My 2001 Cobra trailer arrived from the previous owner with lead weights and dumbbells at the front to increase tongue weight. I removed them and found still plenty of tongue weight and no towing instability behind a Toyota 4Runner at speeds up to 85 mph (Arizona highway speed!) I replaced the tires last year with recommended trailer tires and have absolutely no problems except in very strong gusty side-winds, when I slow to 70 mph.

I suspect the geometry and suspension of the tow vehicle are an important part of the equation - I have always used robust, heavy four-wheel drive vehicles designed for towing and have never experienced severe instability problems.

Mike